FFF 3D Printing

Fused Filament Fabrication

Fused Filament Fabrication (FFF), also known as Fused Deposition Modelling (FDM), is one of the most popular and accessible technologies available in the 3D printing industry. It involves melting a continuous string of plastic filament, usually 1.75 mm or 3 mm in diameter, through an extruder assembly that is capable of moving anywhere within a defined 3D space. Imagine a glue gun mounted on a motorised gantry that can move in the X, Y and Z directions and you will have something very close to a common FFF 3D printer. Except that the glue gun melts plastic instead and the whole contraption is controlled by a computer.

Whilst FFF 3D printers are mechanically simple once you see one in action, it is the software behind them that allows them to materialise your imagination. The machines are designed to accept similar files to that of a CNC router and it creates objects by drawing cross sections of the part, one layer at a time. In the end the object will look like a sequence of MRI scans stacked together, with each layer typically around 0.1 mm to 0.3 mm thick.

The diagram below illustrates a schematic representation of a typical FFF 3D printer. The filament a) is fed through a heated moving head b) that melts and deposits it, layer after layer, in the desired shape c). The print bed e) lowers after each layer is deposited. Sometimes vertical support structures d) are needed to sustain overhanging features.

Source: Paolo Cignoni, Schematic representation of Fused Filament Fabrication 01, CC BY-SA 4.0

Below is a quick time lapse video demonstrating what is physically possible on a FFF 3D printer. 

Video by RepRapPro via YouTube

The biggest advantages of FFF 3D printing are:

  • Accessible with low cost of entry. Machine prices have decreased tremendously over the last few years and will continue to fall. The printers are simple and easy to learn, operate and maintain without a large number of specialised components.
  • Ease of handling. The raw materials are in solid filament form that comes in a spool that is simple and easy to setup. Other technologies use liquid resins or powdered materials which can become messy and require specialised equipment.
  • Large selection of materials. The technology allows for the use of various common and experimental polymers that are available from a wide range of suppliers.

Some drawbacks of FFF 3D printing are:

  • Low resolution. Whilst some high end machines can print at layer heights as fine as 0.01 mm, it is still not as low as other technologies that are currently available such as Stereolithography (SLA) or Selective Laser Sintering (SLS).
  • Poor overhang performance. Parts that have overhang features of 45º or more will generally require support structures to be printed to avoid the molten plastic from drooping down. As a result the process increases material wastage and the surface finish usually suffers. Solutions such as soluble supports are available, where the support material is made from a material that dissolves away in solution once the print is complete. However this requires the machine to be capable of printing with multiple materials simultaneously which presents its own set of challenges.
  • One colour per part. Unless the machine can print with multiple filaments simultaneously or have an automatic filament swapping mechanism, the parts are limited to one colour per part. However this limitation is not exclusive to FFF alone and is common to most consumer grade 3D printers. The industry is actively developing solutions in this space and it should not be long before full colour 3D printing becomes affordable.

Size

With our current capabilities, we can make parts that fit within an envelope of 230 mm(L) x 150 mm(W) x 150 mm(H).

However, bigger components are always possible by making them in sections, such as the model train track below.

This was a replacement track for an old model train set. The piece was divided into 3 to fit within the print volume of our machine. Custom friction joints were designed into the pieces to allow quick assembly without the use of glue or fasteners.

The main challenges with this approach are:

  • Visible seam lines between parts. Can be hidden with traditional post processing techniques such as sanding and filling. Alternatively it may be possible to design seams into the aesthetic style of the object.
  • Fitting tolerance. Some materials are more susceptible to thermal warping than others, as a result parts may not fit together perfectly. Can be mitigated through design in some cases or sanding the mating surfaces.

Materials

There are a wide variety of materials that are currently available for FFF 3D printing. The most popular types are PLA and ABS.

  • PLA (Polylactic Acid), is a biodegradable corn starch based plastic very commonly used for FFF 3D printing due to its low melting temperature (typically around 210 ºC) and dimensional stability. It is non toxic, does not release unpleasant fumes and does not warp easily even without a heated print bed. It usually has a somewhat shiny finish which can be desirable depending on the application. Its weaknesses are that it is not very strong compared to other plastics and can be brittle, also its low glass transition temperature means that it does not hold its shape well under prolonged heat exposure (such as being left in a car on a sunny day). However materials technology is advancing quickly with PLA and variants that can withstand higher temperatures or more stress are becoming increasingly available.
  • ABS (Acrylonitrile butadiene styrene) is one of the oldest plastics to be used in FFF 3D printing. It is petroleum based and extensively used in everyday consumer products. LEGO bricks are the prime example of an ABS product. It is much stronger than PLA, will likely yield before fracture (graceful failure), and generally gives a more matte finish. However it is much more difficult to work with as it requires higher melting temperature (around 240 ºC), releases toxic fumes and is notorious for thermal warping and shrinkage. Therefore parts printed in ABS have to be carefully designed if it is intended to fit together with a specific tolerance.
PLA (left) tend to come out with more glossy finishes than ABS (right)

Generally speaking, PLA is suitable for anything that is designed for aesthetic purposes with no load bearing requirements and ABS is better suited for parts that need some structural strength. We will work with you to find the best solution for your application.

Other less common (but not for long) materials that are available for FFF 3D printing include:

  • PETG (Polyethylene terephthalate glycol-modified) – Strong and stiff, good for spring or snap-together applications. Easy to print but strings easily and supports are difficult to remove. Shiny finish.
  • Metallic PLA – PLA with infused metallic power to produce parts that can be buffed to give a polished metallic finish.
  • Carbon Fibre PLA – PLA infused with carbon chains to increase strength. Parts are matte black in appearance, very stiff but can be brittle if over stressed.
  • Colour changing PLA/ABS – plastic that can change colour either by ultraviolet light or temperature. Some may even glow in the dark.
  • Conductive PLA – Electrically conductive plastic that can be used for electronic applications. The resistance is typically in the kΩ range and is matte black in finish just like carbon fibre.
  • Nylon – Very strong and flexible. Can sometimes be used for end use engineering applications but difficult to print as it has high melting point (up to 260 ºC) and is susceptible to moisture absorption. The filaments commonly need to be stored in vacuum sealed bags and dried in an oven before use.

 

This is by no means an exhaustive list and new materials are developed for the technology everyday. Polymers are one of the most cost effective and versatile materials in the world that is well suited for almost every prototyping application.

Resolution

In the world of 3D printing, resolution generally refers to the height of the layers that make up the print. The thinner each layer is, the higher the resolution. We can make parts at 0.3 mm, 0.2 mm or 0.1 mm layer heights.

0.3 mm (left), 0.2 mm (middle) and 0.3 mm (right) prints

High resolution prints will provide much finer details with less noticeable grain lines. However the higher the resolution, the more layers are required to be printed and the production time increases dramatically. In addition, the FFF process of printing one layer on top of another means you are constantly laying hot plastic on cold plastic, which is not ideal for bonding. Therefore parts are inherently weaker between layers and a high resolution print will have many more weak points due to increased number of layers.

Overhang Support

Prior to restoration works that was completed in 2001, the Tower of Pisa leaned at an angle of 5.5º. Hypothetically, if the tower managed to lean to an angle of 45º without toppling, we would still be able to 3D print a model of the tower. If the angle was even greater than 45º, the print would look terrible as the molten plastic for each layer that overhangs over thin air will droop down under the influence of gravity and lose its shape. One way to fix this is to print in space, but not everyone has that sort of privilege (perhaps a future space experiment? If anyone at NASA or SpaceX is reading this, we would be delighted to collaborate).

From 60º onward, deterioration of underside surface finish becomes apparent. At 70º, the plastic is not even adhered to the body of the part, hence strings can be seen peeling off from the surface.

The more economical solution is to use support structures. The design of support structures are generally automatically generated by software and can look like clusters of thin columns or tree branches. They can also be deliberately designed in to optimise print quality in some cases. The idea is that these structures are printed to gently support overhang surfaces to stop drooping. Most of the time the structure will not fuse with the part and can be teared off afterwards. However it does not completely alleviate the issue and will still leave a rough surface afterwards. The best solution is to avoid harsh overhang surfaces through clever design if possible.

Adding support structure will slightly improve the surface finish of harsh overhangs. But it also requires more work in removal and increases the risk of damaging the part in the process.

There are filaments that are designed to dissolve in solution which can be used as supports that are in full contact with the part. This allows the entire overhang surface to be fully supported and after the print is complete, you submerge the part in the solution and the support melts away. This will provide a much nicer finish but requires a multi-material printer which has the risk of cross contamination and ending up with soluble pockets within the solid part.

Post Processing

Just like building any plastic models, it is possible to apply normal post processing techniques to a 3D printed part to improve its aesthetics. These include but are not limited to:

  • Sanding
  • Priming
  • Filling
  • Painting
  • Polishing

With a bit of patience, a 3D printed part can be nearly indistinguishable from a mass produced, injection moulded product. Except it will be customised and unique.

Chemical Polishing

One of the most interesting post processing techniques that can be applied to ABS printed parts is chemical polishing using acetone. As ABS dissolves in acetone, a controlled exposure can cause surface melting that results in a wet, glossy finish that is smooth, eliminates print lines and make the part look like it’s been buffed to a near mirror shine. It works best for parts that have curved, organic contours.

For certain applications, this treatment can have amazing results!

Parts that have fine details or are designed to fit together with a defined tolerance are not recommended for chemical polishing as the process will round out any sharp features and cause corners to swell up. Also note that once treated, it may take weeks or even months for the acetone to fully disperse from the plastic so the part may continue to slightly deform long after the treatment is complete.